The present invention relates to optical imaging and, more particularly, to systems, processes and products involving episcopic imaging, i.e. imaging utilizing light generally reflected from or at a copy sheet or other visual subject, as distinguished from diascopic imaging, i.e. imaging utilizing light generally directed through a copy sheet. The present invention is directed primarily to large copy projectors, exemplified by so-called "overhead" and "opaque" projectors. Diascopic overhead projectors have been characterized by: bulky hardware that obtrudes between the audience and the projected image and causes the operator to assume an unnatural posture in order not to obtrude between the projection lens and the screen; and transparencies that are uncomfortable for the opertor to view, handle and store. Episcopic overhead projectors theoretically are more compact than diascopic overhead projectors because illuminating source and imaging lens are at the same side of the copy sheet, whereby vertical dimensions are reduced. But, in practice, episcopic overhead projectors also have been obtrusive because either (1) the light source and imaging lens are positioned by a post in a sizeable casing above the copy sheet or (2) a bulky housing envelops the light paths to and from the copy sheet in order to control glare. Episcopic copy, when composed of paper or sheeting of equivalent appearance, has not been adapted for brilliant imaging because of its optical diffusivity.
The primary object of the present invention is the provision of systems, processes and products involving a portable projector and a thin reflectorized plastic copy sheet assemblage that are particularly interrelated to achieve, during projection, direct visual and manual access to the copy sheet by the operator, absolute shielding of the operator and the audience from illuminating light, audience viewing of the projector (together with the operator) and the screen in side-by-side relation, and natural posture of the operator at the projector. The optical projector comprises a low profile housing in which are positioned an upper Fresnel lens underlying the copy sheet, which has its front face upward for direct viewing, a source of illuminating light from which the exterior of the housing is absolutely shielded by the copy sheet itself, and a folded optical path below the Fresnel lens to a wide angle catadioptric reflector at the side of the projector. The catadioptric objective reflector has an extremely wide field angle and a pair of semi-field conjugate surfaces, both on the same axial side of the catadioptric reflector, one of which is disposed at the visual transparency and the other of which is disposed at the viewing screen. The arrangement is such that an unusually powerful illuminating lamp is practicable. The copy sheet is characterized by a lower reflecting face that is accessible to the optical system from below but that can be marked from above by a chemical reactant ink from a pen which is manually held by an operator. Corrections can be made by a chemical etching patch which can clear sections of the lower reflecting face for marking. The thin reflectorized plastic copy sheet is tensioned above the Fresnel lens in order to achieve high quality optical reflective imaging. The tensioning mechanism interacts with the illuminating source so that dimming occurs during copy sheet exchange.
Other objects of the present invention will in part be obvious and will in part appear hereinafter.
The invention accordingly comprises the systems, processes and products, together with their components, steps, parts and interrelationships, which are exemplified by the present disclosure, the scope of which will be indicated in the appended claims.